1. Field of the Invention
The invention relates to a method for producing a silicon single crystal, comprising the following steps:                inductive heating of a silicon plate;        melting granular silicon on the silicon plate; and feeding the molten silicon through a flow conduit in the center of the plate to a phase boundary, at which a silicon single crystal crystallizes.        
2. Description of the Related Art
A method for producing single crystal silicon by melting and crystallization of polycrystalline silicon granules, which will be abbreviated below as GFZ, is described for example in DE 10 2009 051 010 A1. It is a variant of the zone refining (floating zone method, FZ method), from which, however, there are particular differences. The material which is melted in the GFZ method and crystallized to produce a silicon single crystal is granular polycrystalline silicon, so-called granulate, while in the FZ method a feed rod of polycrystalline silicon is used. In order to melt the granular silicon and regulate the crystallization of the single crystal, separate induction heating coils are respectively used, while in the FZ method a common induction heating coil is provided for both tasks. Only in the GFZ method is it customary to use a plate on which the granulate is melted and fed in the molten state through a flow conduit in the center of the plate to the phase boundary, at which the silicon single crystal crystallizes.
In an initial phase of the GFZ method, the still obstructed flow conduit is partially melted from below so that a small volume of molten silicon, a melt drop, is formed. The energy required for melting the obstructed flow conduit is transmitted inductively from an induction heating coil arranged below the plate, the pulling coil. A seed crystal is placed from below on the melt drop and lowered further. At the phase boundary, which is formed at the contact of the seed crystal with the melt drop, after a phase of eliminating dislocations, a silicon single crystal crystallizes. The silicon required for the growth of the single crystal is initially provided only by melting the obstruction of the flow conduit, and later essentially by melting granulate on the plate and by feeding the resulting melt through the now open flow conduit to the phase boundary. The energy required for melting the granulate is transmitted inductively from an induction heating coil arranged above the plate, the melting coil, onto the plate and the granulate lying thereon.
Silicon is a semiconductor, which scarcely conducts an electrical current at room temperature. Correspondingly, inductive heating of silicon is only effective at comparatively high temperatures. In connection with the FZ method, DE 44 16 543 A1 proposes to facilitate the inductive coupling of silicon with an induction heating coil by preheating silicon by means of a susceptor to temperatures at which the electrical conductivity of silicon is much greater than at room temperature. However, the use of such a susceptor is not viable for the GFZ method because its presence would interfere with the crystallization of the single crystal.
The electrical conductivity of silicon at room temperature can be increased by doping the silicon with an electrically active dopant of the p type or n type. Examples of suitable dopants are in particular boron (p type) and phosphorus (n type). The plate can therefore already be heated inductively at room temperature, if it has a correspondingly high dopant concentration.
A disadvantage with the previously described process is that the dopant contained in the plate can pass uncontrolledly from the plate into the single crystal during the growth of the single crystal. Conventionally, however, it is desired that the single crystal and the semiconductor wafers produced therefrom have dopant concentrations which meet a predetermined specification and are not altered by uncontrolled influences.